1. Field of the Invention
This invention relates generally to a device and method to monitor vehicle traffic speed and volume and more specifically to a device and method that detects and uses variations in ambient radio frequency (RF) energy to passively monitor vehicle traffic speed and volume.
2. Discussion of the Related Art
The United States Department of Transportation has sponsored the Intelligent Transportation Systems (ITS) program as part of a major effort underway to improve the nation's transportation system using advanced technology rather than simply adding to the number of roads presently in existence. The National Program Plan for ITS currently identifies twenty-eight user services that form the basis of ITS in efforts and funding. Six of these user services require speed and traffic density information for all major roads in a defined area. Installing traffic speed and density sensors along all major roadways in a metropolitan area understandably constitutes a major expense which would be prohibitive unless inexpensive, reliable and easily serviceable sensors are used.
Many types of traffic sensors have been proposed and tested with varying degrees of success. Currently, two of the most popular technologies for vehicle detection are inductor loops and video cameras. Both, however, are expensive approaches to traffic sensing. Inductive loops are attractive because of their simplicity but their high installation and maintenance costs make them a poor choice for large scale use. Most cities in the United States which employ loop sensors in the traffic management program report that approximately 25-30% of the sensors in the system are operating improperly at any one time. Automatic traffic detection using video cameras is a flexible approach that can be applied to intersections on multi-lane highways except that dependence on video cameras is a complex and costly operation. The cost to instrument a 4-way intersection using either loops or cameras is estimated to be $25,000.00. Providing instruments at a multi-lane section of highway would require approximately the same level of expense
Most of the ITS user services require an up-to-the-minute knowledge of traffic conditions over a wide metropolitan area. Traffic sensors will need to cover all major roads in order to provide information for incident management, pre-trip travel information, en-route driver information, route guidance, traffic control, and emergency vehicle management. The wide-spread installation of traffic sensors on major roads will enable recurring and non-recurring congestion to be detected, monitored, reported, and managed. Delays can be minimized and travelers can adjust travel based on expected delays. For example, the location of an incident can be immediately determined by an abrupt decrease in speed that occurs at an incident. Rescue vehicles and tow trucks can be immediately dispatched to the correct location. The effect of the incident can be monitored by observing the extent of the congestion, the traffic flow approaching and leaving the area, and the speed reductions that occur. Predictions of total traffic congestion and delays for the next several hours can be made based on the traffic measurements, expected traffic flow, time estimates for clearing the incident and traffic models. Travelers can then adjust their immediate travel plans based on the current situation, and make future plans based on normal delays. The ease of measuring traffic patterns will make planning more effective and accurate. All of this will have the effect of reducing the economic, social, and environmental costs associated with recurring and non-recurring congestion.
There are many different types of traffic sensors available. There are also different requirements for traffic sensors based on where they will be installed and how they will be used. For example, sensors used for signal control need to be very accurate at detecting the presence of vehicles but may not need to measure speed. Sensors used for wide area incident detection need to give an approximate measure of speed and traffic volume at a very low installed cost. Inductive loops are commonly used for detecting vehicle presence but the cost and dangers associated with installing loops as well as their high maintenance cost make them unacceptable for wide area traffic monitoring.
An additional factor when looking at the cost of a sensor is the cost of supplying power and communications to the sensor. These two items can be the main cost in installing a sensor. It is therefore desired to have a sensor that requires only a small amount of power that can run on solar/battery power, eliminating the need for additional power lines. Communications for the sensor can be provided by a wireless network of some sort, assuming the sensor only reports occasionally and when an anomalous condition is detected. Considering the total cost of installation, a low power sensor with wireless communications can be significantly less expensive than other sensors.